This invention relates to a method of producing electrically conductive pastes (herein referred to simply as "conductive pastes") which can be useful as a conductive material for the production of multi-layered ceramic substrates, as well as conductive materials themselves produced by using such a paste.
Ceramic substrates are being used commonly for mounting many kinds of electronic components to form an electronic circuit in order to produce compact electronic devices and instruments. In order to further increase the mounting density, it is becoming a common practice to stack a plurality of ceramic green sheets with circuit patterns formed on their surfaces by using a paste containing a conductive substance and to form an integral multi-layered structure by a firing process. In order to make electrical connections between such ceramic sheets, viaholes are initially formed through the ceramic green sheets by using a drill or a puncher and are filled with a conductive paste. Circuits are formed on the surfaces of the green sheets, for example, by a screen-printing process by using such a conductive paste. After the green sheets are thus prepared, they are stacked one on top of another, compressed, cut to an appropriate size and then subjected to a firing process. The conductive paste, both inside the viaholes and on the green sheets, is sintered at the same time such that the circuits inside the multi-layered ceramic substrate become connected.
As electrically conductive particles which are mixed in to form such pastes, use is frequently made of Cu because it has low specific resistance, because migration does not occur easily and also because it is inexpensive. Pastes, which are made by mixing and dispersing Cu particles in an organic vehicle having a resin component, say, of ethyl cellulose resin, are now in use.
In summary, prior art multi-layered ceramic substrates are produced by subjecting ceramic green sheets and a Cu paste to a firing process simultaneously. In the firing process, however, the ceramic green sheets and the Cu paste shrink differently in that the shrinkage of Cu due to the sintering takes place before that of the ceramic and at a lower temperature. As a result, delamination may take place between the multi-layered ceramics and the conductive material between the layered ceramics or cracks may appear after the sintering, thereby affecting the reliability of the multi-layered ceramic substrates adversely.